Add rig to already weighted model
if the model already has weights, it should have vertex groups. Make sure the bones in your rig are named the same as the existing vertex groups. Then select the model, then the rig, and hit Ctrl-P > Armature Deform (and make sure the armature modifier on the mesh is set to Bind to Vertex Groups). If you have additional bones that you want to use that don't have pre-existing vertex groups, you can enter pose mode on the rig, then enter weight paint mode on the mesh, then select the bones you want to have weights for, press W and select Assign Automatic From Bones. I hope this helps!